Photochromism is a reversible action of a compound which quickly changes the color when it is irradiated with light containing ultraviolet rays, such as the sunlight or the light of a mercury lamp, and resumes its initial color when it is no longer irradiated with light and is placed in a dark place. The compound having the above properties is called photochromic compound and is used as a material of photochromic plastic lenses.
The photochromic compound used for the above application must satisfy such properties that:
(1) The compound has a low coloring degree (hereinafter referred to as initial color) in a region of visible light of before being irradiated with ultraviolet rays;
(2) The compound exhibits a high coloring degree (hereinafter called color density) when it is irradiated with ultraviolet rays;
(3) The compound enables the color density to increase to a saturation at a high rate, i.e., has a high color-developing sensitivity after it is irradiated with ultraviolet rays;
(4) The compound returns to the initial state at a high rate (hereinafter referred to as fading rate) after it is no longer irradiated with ultraviolet rays;
(5) The compound has a high recurring resistance in reversible action by light; and
(6) The compound disperses well in a host material that is used or, concretely, the compound dissolves highly densely in a monomer composition that becomes the host material after cured.
Further, it has been desired that the photochromic plastic lenses develop a neutral tint such as brown or amber as a color tone in the state of developing color. Therefore, what color the photochromic compound will develop is a very important factor, as a matter of course.
When a desired color tone cannot be realized by the use of a single photochromic compound, the color tone is adjusted by mixing together a plurality of photochromic compounds that develop different color tones. In this case, it is important that the individual compounds that are mixed together have excellent properties and, besides, that the whole compounds (mixture) maintain balance in the properties.
From the standpoint of adjusting the color tone, it is important to use a photochromic compound which by itself develops a yellow color or a neutral tint. As the above compound, prior art 1 discloses a chromene compound [compound (A)] represented by the following formula (A), and the prior art 2 discloses a chromene compound [compound (B)] represented by the following formula (B). According to these prior arts, photochromic plastic lenses having favorable photochromic properties are obtained by using a curable composition which is obtained by dissolving the above photochromic compounds in a radically polymerizable monomer, and by molding (cast polymerizing) the curable composition by being cured by the thermoradical polymerization.

The method of producing the photochromic plastic lenses by the cast polymerization employed by the prior arts 1 and 2 (also called “in-mass” method or kneading method) is a representative method of producing photochromic plastic lenses imposing, however, a limitation on the kinds of the polymerizable monomers that can be used for obtaining favorable photochromic properties.
In recent years, attention has been given to a coating method as a method of producing photochromic plastic lenses without the above limitation (see prior art 3). According to the coating method, a coating agent comprising a polymerizable and curable composition containing a photochromic compound is applied onto the surface of the lens material, and the applied coating is cured to form a photochromic coating to thereby impart photochromic property to the lens material. Therefore, if a favorable coating adhesion is obtained, there is, in principle, no limitation on the lens material.
[Prior art 1] U.S. Pat. No. 5,783,116
[Prior art 2] Leaflet of International Laid-Open WO00/15628
[Prior art 3] Leaflet of International Laid-Open WO03/011967